Study genetic factors required for expression of transformation by tumor viruses by obtaining and analyzing morphologic revertants of MSV nonproducer cells which contain normal sarcoma viruses. Isolation and characterization of murine sarcoma virus mutants temperature sensitive for phenotypic expression of the transformed state. Determine the role of murine leukemia virus antigens in stimulating host immunity to experimental solid tumors. Study in vivo host regulation of genetic expression of a horizontally transmitted leukemia virus.